Acoustic diaphragm



April 28, 1970' RRQBBJN v 3,508,626

AG OUST I C DIAPHRAGM Filed Dec. 22, 19s? ZSheets-Sheet 1 PIC-.5.

\DEAL SQUARE WAVES.

RISE TIME ACTUAL SQUARE WAVES 400 HZ. 1000 H2. 5000 H2.

United States Patent U.S. Cl. 181-32 17 Claims ABSTRACT OF THE DISCLOSURE Acoustic diaphragms, such as loudspeaker diaphragms, are heavily coated with mineral or metal particles in an adhesive binder, whereby acoustic performance quality is greatly improved.

This application is a continuation-in-part of my copending applications, Ser. No. 356,640, now abandoned, filed Apr. 1, 1964, entitled Acoustic Diaphragm and Ser. No. 356,662, filed Apr. 1, 1964, entitled Acoustic Diaphragm.

This invention relates to electro-acoustic transducers and more particularly to a loudspeaker diaphragm and methods of manufacture thereof.

The design of single member transducer diaphragms for loudspeakers intended to perform with equal efficiency, good transient response, and low distortion over substantially the entire audio frequency range, presents problems which heretofore have been almost impossible to solve. inasmuch as the ultimate aim of achieving simultaneous generation of the extremes of the high and low ends of the frequency spectrum have required mutually contradictory solutions.

It has been generally accepted prior art theory that, for efficient radiation of the lower frequencies, below piston diameter, which is the frequency at which the diameter of the diaphragm is /3 of the wavelength, the diaphragm should be large in area and move as a whole in true piston fashion. To avoid or minimize creation of sub-harmonics, cone-cry, and distortion caused by radial bending or flexure, it should be as stiff or rigid as possible.

However, to achieve such a large, rigid diaphragm with known material results in either a structure of high mass, which due to high overall inertia inhibits the reproduction of higher frequencies, or in a low mass rigid structure of multicellular design, as in polystyrene foam cones, which absorbs high frequencies due to momentary collapse of interconnecting cell walls.

Further, in order to realize the lowest possible frequency response it is desirable to have a low mechanical resonance, as response drops off rapidly below that point. Since this resonant frequency is due to a combination of cone mass vs. suspension compliance, a low frequency resonance can be attained in either of two known ways. (1) by increase of the mass factor with the compliance remaining the same, or (2), by increase of the compliance factor with the mass remaining the same. If solution (1) is used, there is a practical limit to the amount of mass that can be added before overall efficiency is impaired, plus the undesirable contribution of excess inertia which leads to deterioration of high frequency generation. If solution (2) is used, there is also a practical limit to the degree that compliance can be increased before mechanical instability, in relation to alignment within the voicecoil magnetic gap, sets in.

Yet for the higher frequencies above piston diameter prior art design theory maintains that low mass diaphragm structure is necessary to provide the low inertia required for propagation of these frequencies. And further, that there must be some radial flexibility to encourage sym- 3,508,626 Patented Apr. 28, 1970 ICE metrical mode bending of the diaphragm, which behavior in this mode produces high frequency transmission.

Thus, the design requirements, based on prior art are virtually unattainable in a single diaphragm structure unless severe compromises in the desired goals are made. These compromises take the form of large area, low mass diaphragms formed of relatively stiff paper or other substances, which serve to minimize low frequency radial subharmonics, yet retain sufiicient flexibility for bending in a symmetrical mode behavior at higher frequencies. However, the low overall mass characteristic results in a rela tively thin walled structure, subject to considerable deformation under transient impulses, causing cone break up, oil canning, edge flutter, upper high frequency absorption, and producing a high order of distortion. In addition, the thin, stiff walls have a very low internal damping factor which creates ringing, production of spurious responses, and, by storing energy, impart a slow decay envelope to impressed signals. If an attempt is made to damp the cone externally by cementing damping materials such as felt, polyurethane foam, etc. to the cone surface, additional high frequency absorption results, which far outweigh any benefits derived.

Another vexing inconsistency in speaker design occurs in the shape of the diaphragm itself. In order to increase the high frequency response it is desirable to form the diaphragm into a configuration having non-linear sides which may be described by a suitable mathematical curve, for example, an exponential or hyperbolic curve. Such shapes allow for progressive decoupling of the diaphragm mass as the high frequencies presented to the diaphragm are increased. But such shapes, using conventional materials, while providing high frequency bending action, reduce the radial rigidity necessary for good low frequency piston action. Thus, the general trend for high quality audio reproducing systems is to provide straight sided, high mass, very rigid piston-like diaphragms for low frequency woofer speakers, and low mass, relatively rigid, conical, exponentially or other mathematically curved diaphragms for mid-range and tweeter reproduction, thereby necessitating the use of several speaker units for each system in order to cover the entire audio frequency.

It is therefore an object of the invention to provide an improved, high efiiciency, single diaphragm, wide range loudspeaker having a substantially constant frequency response characteristic over the entire audio range.

It is, another object of the invention to provide an electro-acoustic diaphragm for reproducing a wide range of audio frequencies, having non-linear sides of great rigidity.

It is a further object of the invention to provide an electro-acoustic diaphragm having a relatively large working mass with great rigidity, yet capable of reproducing the high frequency range of the audio spectrum with negligible reduction in output over that of the low frequency range of frequencies.

It is still another object of the invention to provide an electro-acoustic diaphragm for loudspeakers having a high working mass and high damping characteristics but exhibiting a smooth, substantially unattenuated response over the entire audio frequency range.

It is yet another object of the invention to provide an electro-acoustic diaphragm for loudspeakers having novel means for generating high frequency sound radiation.

Other objects, features and advantages of the invention will become apparentfrom the following description of certain embodiments thereof, taken in conjunction with the accompanying drawings in which:

FIG. 1 is an axial cross-section through a diaphragm in accordance with the invention for use in a moving coil loudspeaker;

FIG. 2 is an enlarged fragmentary portion of the cross section of FIG. 1;

FIG. 3 shows diagrams of output waveforms when square waves at several frequencies are applied to a diaphragm in accordance with the invention; and

FIGS. 4-7 are frequency response curves of several acoustic diaphragms.

All of the several embodiments of diaphragms contemplated by the invention are formed from materials which have been found to have highly advantageous characteristics for loudspeaker diaphragms. Such materials not only provide superior operating characteristics, but also are adaptable to a wide variety of design variations and high speed production techniques.

In accordance with the first embodiment of the invention, the novel diaphragm material comprises a mixture of a hard, inert inorganic material in a granular, powdered, or other finely divided form and a binding material which upon solidifying, serves to bind the powdered material into an apparently solid mass, but which retains a resilient character.

I have found that inert, hard mineral materials, such as plaster of Paris, quartz, silica, clay, feldspar or other ceramic and vitreous material, flint, glass fibers, emery, boron, carbon, and gypsum, for example will work in my invention. A preferred mineral material is powdered pumice, whereas another mineral material which provides excellent results is alpha-gypsum, and melamine, commercially known as hydromite, and manufactured by the U.S. Gypsum Company. The selected hard materials should be in non-grain oriented form, such as particles, granules, tiny beads, powder, short fibers in the case of glass, asbestos, and rock wool fibers, or any other form which may be incorporated in a random arrangement in a non-homogeneous combination with a flexible, soft, binder material.

In accordance with another embodiment of the invention, the novel diaphragm material comprises a mixture of metal particles in a granular, powdered, or other finely divided form and the binding material which upon solidifying, serves to bind the powdered metal into an apparently solid mass, but which retains a resilient character. The metal particles may be mixtures of different metals or metallic alloys, if desired.

I have also found that metals in particle form such as aluminum, iron, copper, magnesium, nickel, tantalum, tin, zinc, manganese, rhodium, silver, gold and oxides and alloys thereof, for example, will work in my invention. The selected metals also should be in non-grain oriented form, which may be incorporated in a random arrangement in a non-homogeneous combination with a flexible, soft, binder material.

Suitable materials for the binder, for either the metal or mineral particles, include polyvinyl acetate, polyvinyl alcohol, epoxys, phenolics, urea or melamine formaldehyde, cellulose acetate, nylon, vinyl resins, or other addition or condensation polymers or mixtures thereof or animal, protein or vegetable glues. However, any material may be used as a binder which solidifies upon drying with a minimum shrinkage, yet retains a resilient character, and is capable of holding in semi-solid suspension the metal particles or particles of harder mineral material with which it is mixed.

When a polyvinyl acetate emulsion is to be used as a binder material, I prefer to use Elmers Glue manufactured by the Borden Co. of New York, NY. or Sobo Glue manufactured by Sloman Mfg. Co., New York, N.Y., both having the consistency of thick cream. The chemical characteristics of the binder material are not important. However, certain physical characteristics of the binder are important, viz, the material when applied should be in a liquid state; it should not shrink upon drying; it should not become brittle; and should not shrink, swell or distort while drying.

To obtain the unexpected results described hereinafter, the proportions between the inert, mineral material and binder material should be greater than 50% mineral material by weight, and less than 50% binder material by weight after the mixture has dried to its final form. However, best results are obtained when the ratio is considerably higher. In one practical example, described hereinafter, the proportions of a mixture of powdered pumice to a polyvinyl acetate binder of Elmers Glue when dry, were in the ratio of about pumice by weight to about 5% polyvinyl acetate by weight. If desired the mixture may be as high as 99% hard material to 1% binder, by weight, or even higher provided there is suflicient binder material to insure a permanent bond and maintain adhesion to prevent flaking or cracking of the coating.

The chosen quantities of a selected hard material, preferably in granular form, and a selected binder emulsion are mixed with a small amount of water or other volatile liquid such as alcohol sufficient to form a slurry having a creamy consistency which is then usually applied to the interior surface of a loudspeaker diaphragm preferably formed from paper, cambric, coarse fibers, or any other conventional diaphragm structure, which henceforth serves as a backing member for the novel material of the present invention.

The mixture may be applied by brushing, spraying, dipping, pouring or any other suitable method onto the inner, outer or both wall surfaces of the pre-fabricated diaphragm. If desired, the mixture may be diluted with, for example, water, or other solvent, sufficiently to facilitate spraying, and may be applied in several layers at different times.

The volatile components are then permitted to evaporate at room temperature, which may take several hours. However, the drying time may be accelerated by heating at C. from 15 minutes to 1 hour either in an oven or by exposure to a heat lamp. While the coating may be applied to the diaphragm surfaces in one or more layers to any desired thickness, I have found that suitable thicknesses for a discrete surface coating are between .1 millimeter and as high as 10 millimeters, with excellent results being obtained when the coating is about 1 to 1.5 millimeters thick. When the coating has hardened, it forms a unified and bonded, homogeneous laminate with a hard, porcelain-like finish.

It is desirable that a sufi'icient quantity of the coating material whether formed from mineral or metal particles, be applied to increase significantly the total weight of the diaphragm. In one practical example, excellent results were obtained when the dry weight of a loudspeaker diaphragm having a nominal mouth diameter of 10 inches was increased from an original 7% grams to 24 grams by application of a mixture of powdered pumice, Elmers Glue and water.

Referring now to FIG. 1, wherein there is illustrated a structural embodiment of the invention, 10 indicates generally a sound radiating diaphragm for a moving coil loudspeaker coated in accordance with the principles of the present invention. Diaphragm 10 comprises a conical portion 12 terminating outwardly in a flexible surround portion 14, preferably corrugated in form as shown. The outer edge of surround portion 14 is secured, as by adhesion, to a suitable mounting ring 16 of a conventional loudspeaker frame. A customary voice coil winding 18 is secured, at the inner, apical end of conical portion 12 to cylindrical portion 20 thereof. A damping paid 22 is fastened by a suitable soft cement 24, such as a neoprene based cement to prevent rigidizing of the pad (FIG. 2), to a dust cover 26 which, in turn, is aflixed to cylindrical portion 20, as by a hard glue 28, such as a cellulose based cement. Damping pad 22 is a conventional element of loudspeaker construction and serves to prevent radiation of out-of-phase signals in the 5 to 7 kilohertz band.

Surround portion 14 is preferably treated with a conventional edge softening solution such as glycerine to maintain a desirable soft surround condition, and it may also be coated with a conventional silicone base damping compound to further improve the frequency response of the diaphragm by providing an outer resistive termination that absorbs mid-range wavelengths and so prevents out-of-phase reflections back into the cone body that would cause cancellation at those frequencies.

The interior surface of cone 12, shown in more detailed in FIG. 2 as being formed from paper or coarse fibrous material, is covered with the coating material 30 of the present invention, comprising a suitable binder 32 which holds mineral or metal particles 34 in suspension. Coating material 30 is applied in the manner and in a selected thickness as described above.

The addition of the high mass of the coating to a conventional paper diaphragm will substantially lower the mechanical resonant frequency of the diaphragm, thereby increasing the bass frequency response of the speaker with which it is used. In the example of the inch speaker described above, its resonant frequency was decreased from 70 cycles to 35 cycles, thereby providing an additional octave of bass response. Thus, one of the criteria for a high quality woofer is achieved, namely, lowering the mechanical resonant frequency of the loudspeaker.

Another advantageous feature of the present invention is that the high diaphragm wall stiffness attained after the coating material applied thereto has dried, provides excellent diaphragm piston action over the range of frequencies, commencing with the piston diameter frequency downward to the resonant frequency of the motor-diaphragm structure. The piston diameter frequency is, of course, the frequency at which the wave length of the transmitted sound is equal to about three times the diameter of the diaphragm mouth.

Another advantage of the rigid diaphragm is its virtual insensitivity to external vibrations not produced by the voice coil, thereby allowing it to be mounted in an unlined speaker cabinet or housing, and yet not exhibit spurious responses due to reflections set up within the housing. Furthermore, as the diaphragm coating material upon drying becomes very hard and rigid, bending of the diaphragm in a radial mode fashion with its attendant distortion, production of sub-harmonics and cone breakup, is resisted and reduced to a minimum. Its use in conjunction with a loudspeaker diaphragm having walls diverging outwardly with an exponential or other non-linear flare are obtained and yet the inherent weakness in a pre-bent structure of this type with its tendency to allow bending and radial mode operation is substantially eliminated.

A further unexpected advantage achieved by the invention is the greatly increased ability of a loudspeaker to which the novel coating is applied to handle much higher signal input power levels. For example, the 10 inch speaker mentioned above had a 3.16 ounce voice coil magnet and a 10 watt peak power rating. After the coatingwas applied, it was able to handle 50 watts of audio power without any discernible cone breakup or distortion.

While the low frequency response and operation of a diaphragm to which the mixture of the present invention is applied, is greatly enhanced, yet the high frequency response is likewise. benefited, which is contrary to normal loudspeaker theory. According to conventional theory, the addition of a large mass to a speaker diaphragm should cause loss of transient response and the serious roll of the high frequency response above the piston diameter frequency of a given loudspeaker. Nevertheless, I have found that so called hard paper diaphragms of the type presently used in many wide range single diaphragms are actually highly compressible and absorbent and thus have a poor high frequency response, but when coated with one of the materials of the present invention, exhibit substan tially uniform frequency response from the bass frequen cies well up into the to 20,000 cycles region with a minimum of peaks and valleys in the response curve.

The advantages and improvements of the present invention are perhaps best illustrated by the waveforms of FIG. 3 which show the results of the application of square waves of different frequencies to a loudspeaker having a diaphragm constructed in accordance with the invention. The loudspeaker model used for this test and the loudspeakers used for all other tests described herein and shown in FIGS. 4-7 are inexpensive, 12 inch cone diameter, conventional production loudspeakers manufactured by Quam-Nichols Corp, Chicago, Ill. This loudspeaker has a semi-exponential stiff paper cone weighing about 12 grams, a 10 ounce ceramic magnet, a 1 inch voice coil, 8 ohms nominal impedance, and a 10 watt power rating at 400 hertz.

All test loudspeakers were mounted in a totally enclosed, air tight, 1.6 cubic foot internal volume cabinet, with a baffle board and frontal radiating area of the loudspeaker forming one side thereof. A hole was placed in the baffle board to allow for air pressure relief. All test loudspeakers were driven at a low level of about one watt, by an amplifier having a substantially flat frequency response. A calibrated dynamic microphone was mounted on the major axis of the test diaphragm, about 12 inches from the mouth thereof. The microphone signal output was fed through another substantially flat amplifier to a high impedance input voltmeter. The signal magnitude for selected frequencies throughout the audio spectrum, as indicated in db by the voltmeter, was recorded and the curves of FIGS. 4-7 were prepared therefrom.

The 12 inch loudspeaker used to produce curve A of FIG. 3, curve D of FIG. 4 curve F of FIG. 5, curve H of FIG. 6, and curve L of FIG. 7 had its diaphragm coated with a mixture comprising 20 grams of powdered pumice, FF grade (Rainbow brand, manufactured by Murray Williams Color and Chemical Co., Maplewood, N.J.), 6.6 grams of Sobo Glue and 6.6 grams of water. A small amount of black dye was added for the sake of appearance. The mixture was then applied and dried in the manner previously described. After the coating material was applied, the surround of the cone was softened and also coated with a conventional silicone damping compound as described previously in connection with FIG. 1.

The square waveforms shown in FIG. 3 are sketches representative of waveforms appearing on an oscilloscope connected to the output of the test microphone amplifier, and show the results when square waves of 400, 1000, and 5000 hertz respectively are applied to the voice coil of the coated loudspeaker, curve A, and an uncoated loudspeaker, curve B. Since square wave testing is indicative of the quality of the frequency and transient response of a loudspeaker, it is readily apparent that loudspeaker A of the present invention has greatly improved transient and frequency response over loudspeaker B. At 5000 cycles, the square wave output of loudspeaker A is still -Well defined, whereas both transient and frequency response of speaker B has greatly deteriorated.

The greatly improved results achieved by my invention are further clearly shown in FIG. 4 wherein there is shown for comparison purposes: the frequency response curve of an untreated loudspeaker, curve B; the frequency response curve of a loudspeaker having its diaphragm coated with the 20 grams of pumice and Sobo Glue mixture, as mentioned above, curve C; and the frequency response curve of a loudspeaker with a diaphragm having the coating of the curve C diaphragm, and in addition the conventional edge softening and damping treatment, curve D. It is clearly seen that the frequency response of the untreated loudspeaker varies greatly throughout the audio spectrum, that the response falls off rapidly below hertz, and that the response has high amplitude peaks in the 2-7 kilohertz range with very rapid fall off thereafter. In contrast, curve D shows that a loudspeaker constructed in accordance with the invention and having a treated edge has an essentially fiat frequency response from less than 50 hertz to over 12 kilohertz. The effect of edge treating to eliminate resonant effects around 2.3 kilohertz is illustrated by the difference between curves C and D.

Although the reasons for the unusual behavior of a diaphragm coated or formed from the material of the invention are not completely understood, the following is believed to be one explanation for its unique operation. When the diaphragm is driven at frequencies having a wave length exceeding the radial length thereof, the driving pressure compacts the entire diaphragm and its coating of hard particles and soft binder into a rigid mass or piston. However, for wave lengths shorter than the radial length, only that part of the diaphragm in- Volved as a function of the wave length is compressed and rigidized, with the remainder of the diaphragm not in use acting as a mass restraint, both hastening the rigidizing of the small portion involved and acting as a hydraulic damping body due to its combined compliance and mass. The small portion of the diaphragm involved is alternately compressed and elongated by each full cycle of impressed signal. During the compression half of the cycle, the small operating portion of the diaphragm is compressed and caused to bulge outwardly in a lateral direction with respect to the diaphragm mouth, creating a compressive wave in the surrounding air. Because of this action, a large number of the hard particles which are in effect suspended in a deformable binder, tend to compress into a rigid interlocked mass and cause the binder to flow out from among the particles. Thus during the compressive portion, the hard particles, in effect, interlock to form an ideal piston for the frequency of the applied signal.

As the diaphragm passes through a null position and then elongates as the rarefaction cycle is described, the cone material is likewise elongated causing the deformable binder to flow from between the particles, with consequent separation thereof to produce a rarefaction wave in the surrounding air, then once again returning to normal position and shape. During this action, it is believed that the larger mass of the diaphragm is not in operation, but remains virtually immovable due to its high inertia. Therefore, it would appear that the driving means for the diaphragm causes pistons of progressively smaller mass to be selected, with such pistons being determined as a function of the wave length of the signal involved. Thus, the high mass of the total cone is apparently a series of closely coupled, intimate masses and compliances of infinitely variable nature, activated and controlled by the frequency of the impressed signal. It would seem that the diaphragm mass behaves hydraulically and yields slightly at its boundary abutting the area involved in the vibration forced by the wavelength of the impressed signals, but is rigid throughout the inactive remaining portion of its body to behave like an immovable solid.

It would further appear that the lack of ringing or tendency to exhibit a resonant frequency in the range between its fundamental low frequency resonance and 20,000 cycles is due to the damping action of the resilient binder material which, while allowing the hard particles to vibrate in the elastic body thereof, to thereby allow the high frequencies to be transmitted rather than absorbed as in softer diaphragm bodies, yet damps out any tendency of the diaphragm to store energy and release it after the energizing signal has ceased.

It is possible that the high frequencies are also transmitted by setting each individual particle into vibration resulting in a particle-to-particle movement within the semi-flexible binder medium, so that a molecular transmission action occurs. In any event there is a noticeable lack of absorption of the high frequencies when diaphragms are coated according to the teachings of the present invention.

The following examples are illustrative only of methods and diaphragms embodying the invention.

8 EXAMPLE 1 In a preferred example, a pumice and polyvinyl acetate coating for a 10 inch diameter loudspeaker, having a medium stiff, exponential flared, paper diaphragm provided excellent results and produced a smooth response curve, similar to curve I, FIG. 6, with extended low frequencies and high frequency ranges above 15 kilocycles. A mixture comprising about 22.7 grams of grade FF pumice powder, about 7.4 grams of an addition polymer such as a polyvinyl acetate aqueous emulsion, for example, Elmers Glue, and approximately an additional 4 grams of water were stirred together to form a slurry Which was applied as by brushing, to the loudspeaker diaphragm, which had an average, effective surface area of about 420 sq. cm.

When the water had evaporated and the residue had become completely dry, the applied mixture formed a hard, smooth ceramic-like surface about 1.5 mm. thick, and tightly adhered to the paper cone surface. The total dried weight of the added material was 24 grams, or about .055 gm./sq. cm. which added to the original 7 /2 gram of the paper diaphragm, resulted in a loudspeaker having a total diaphragm weight of about 31 /2 grams. The amount of pure polyvinyl resin remaining after drying apparently is about 1.3 grams. Therefore, the ratio of binder to hard material in this example is about 1 to 17; or about 5 /z% of the total coating weight is binder material.

The results obtained in this example were further improved by forming the 22.7 grams of the hard material from a mixture of about 50% grade FF powdered pumice and 50% carbon black, with the particle size of the pumice being about '6 times the size of the lamp black particles. Evidently the smaller lamp black particles tend to fill the interstices between the larger pumice particles according to Macadams packing density principle thereby providing a more rigid coating.

For an 8 inch diameter speaker having a paper diaphragm weighing approximately 4 grams and an average effective surface area of about 215 sq. cm., a mixture comprising about 14 grams of grade FF powdered pumice, approximately 5 grams of polyvinyl acetate aqueous emulsion such as Elmers Glue, and about 2% grams of Water was applied to the diaphragm by brushing to add about 15 grams weight thereto, when dry or about .07 gm./ sq. cm. The unusual results provided by the present invention were achieved in this case also. Likewise, about 30 grams, dried weight, of this mixture were applied to a 12 inch, exponentially flared speaker diaphragm at a density of about .05 gm./sq. cm. to produce the same unusually good results.

EXAMPLE 2 To a 10 inch loudspeaker having a medium hard paper diaphragm with exponential flare, a mixture comprising about 18.8 grams of Hydromite (essentially 75% alpha gypsum and 25% melamine by weight), a 1:1 by volume mixture of polyvinyl acetate aqueous emulsion such as Elmers Glue or Sobo Glue and water weighing about 7.4 grams was applied, as by coating with a brush, to provide 26.2 grams total. A small amount of citric acid, such as .6 gram to 500 grams of water was added to retard the setting of the mixture.

For an 8 inch diameter speaker, a suitable coating mixture was found to comprise about 15 grams of Hydromite and about 5 grams of a second mixture comprising equal parts of water and the polyvinyl acetate aqueous emulsion.

It has been found that the proportion of binder material to hard material when dried is, for best results, in the range of 5% or 6% by weight. The upper limit is reached when so little binder is used that insuflicient adhesion of the hard particles occurs, and they tend to flake or crack off. The lower limit is reached when there is so much dried binder material (over about 50%) in proportion to the hard granular material that the binder functions essentially as a high frequency damping medium, causing severe roll off, because the particles are so widely spaced within the damping materials that the cone cannot act in accordance with the principles of the invention.

While the proportions listed in the above examples may be varied and still provide satisfactory results, the total amount applied to a selected speaker should be limited only by the reduction in over-all efficiency in relation to output vs. input power that can be tolerated as the weight of the diaphragm increases. As set forth above, excellent results were obtained with some speakers when the weight of the material was at least three times the uncoated weight of the diaphragm to which it was applied. Of course the coating may be as thin as desired. However, until the coating is at least as heavy as the uncoated diaphragm, improvements in the operation of a treated speaker will not become apparent. For nominal 8, and 12 inch speaker diaphragms, coating densities between .02 and .1 gm./sq. cm. provide good results, with optimum results occurring at densities of about .06 gm./sq. cm.

The effect of coating thickness on frequency response, expressed as differences in Weight of the coating material is shown in FIG. 5, wherein curve E represents the frequency response of the 12 inch test loudspeaker coated with a mixture of grams of grade FF powdered pumice, 5.0 grams of Sobo Glue, and 5.0 grams of water. Curve F represents the frequency response of the 12 inch test loudspeaker coated with the same mixture as speaker E except that the pumice weight was increased to grams, the binder to 6.6 grams, and the water to 6.6 grams. Curve G represents the frequency response of the 12 inch test loudspeaker coated with the same mixture as the speaker of curve E except that the pumice weight was increased to 60 grams, the binder to 20.0 grams and the water to 20.0 grams, resulting in a coating thickness of about 4 mils. Each speaker had its diaphragm edge surround treated in the manner described previously. Accordingly, it will be seen that the curve of speaker E begins to improve on the curve of untreated speaker B of FIG. 4 with a lower frequency roll off at about 75 hertz and a much smaller high frequency roll off above 7.5 kilohertz. Curve G shows an expected rise in low frequency response, fall off in frequency response above 5 kilohertz, and a generally uneven response over the entire audio spectrum.

FIG. 6 shows the effect on frequency response of the 12 inch test speaker when coated with a mixture comprising 20 grams of grade FF powdered pumice, 6.6 grams of water, as before, but with 6.6 grams of different binder materials. The binder material for the speaker coating mixture of curve H was Sobo Glue; for the speaker of curve I, Elmers Glue was used; and for the speaker of curve K it was Hyplar Medium, a high copolymer plastic latex emulsion manufactured by M. Grumbacher, Inc. of New York, NY. It will be noted that best results are obtained with the Sobo Glue binder, although the response curves for the other two binders of curves I, K are very good.

While the pumice used for the coatings of the test speakers of FIGS. 3-7 was grade FF, equally good results were obtained when 20 grams of FFFF grade pumice were used with 6.6 grams of Sobo Glue and 6.6 grams of water to form a coating material with which a 12 inch test speaker was coated.

Some additional examples of diaphragms embodying metals as the powdered ingredient of the coating material of the invention are as follows.

EXAMPLE 3 A powdered bronze and polyvinyl acetate coating for a 10 inch diameter loudspeaker, having a medium stiff, exponential flared, paper diaphragm provided excellent results and also produced a smooth response curve with extended low frequencies and high frequency ranges above 15 kilocycles. A mixture comprising about 22.7 grams of bronze in a finely divided or powdered form, about 7.4 grams of an addition polymer such as the Elmers Glue polyvinyl acetate aqueous emulsion, and approximately an additional 4 grams of water were stirred together to form a slurry which was applied as by brushing, to the loudspeaker diaphragm, which had an average, effective surface area of about 420 sq. cm.

When the water had evaporated and the residue had become completely dry, the applied mixture formed a hard, smooth ceramic-like surface about 1.5 mm. thick, and tightly adhered to the paper cone surface.

EXAMPLE 4 For an 8 inch diameter speaker having a paper diaphragm weighing approximately 4 grams and an average effective surface area of about 215 sq. cm., a mixture comprising about 14 grams of powdered bronze, approximately 5 grams of polyvinyl acetate aqueous emulsion, and about 2% grams of water was applied to the diaphragm by brushing to add about 15 grams weight thereto, when dry or about .07 gm./sq. cm. The unusual results provided by the present invention were achieved in this case also. Likewise, about 30 grams, dried Weight, of this mixture were applied to a 12 inch, exponentially flared speaker diaphragm at a density of about .05 gm./ sq. cm. to produce the same unusually good results.

EXAMPLE 5 A mixture comprising 20 grams of powdered aluminum, 1-511 manufactured by Reynolds Aluminum Co., New York, N.Y., having a particle size of about 13 microns (between 200 and 325 mesh), 6.6 grams of Soho Glue and 6.6 grams of water was applied to the 12 inch test model speaker, described above. The unusual results provided by the present invention were again achieved, as shown by the response curve of this speaker, FIG. 7, curve M. For comparison purposes, curve L, showing the frequency response of the 12 inch test speaker coated with 20 grams of pumice powder, is included in this figure.

FIG. 7 also shows the frequency response curve N for a coated 12 inch test speaker in which the major coating ingredient is small squares of aluminum, about & inch on a side, known as Glitter, and manufactured by Belmont Industries, Inc., Chicago, Ill., and the binder material is a mixture of 6.6 grams of Sobo Glue and 6.6 grams of water. It will be noted that the frequency response curve for this speaker is slightly smoother in the 1 to 5 kilohertz region than response curve M.

As the material of the present invention is so easily applied by brushing, spraying, dipping, pouring or any other suitable method onto the surface of an already formed paper, cambric, or other standard type of diaphragm structure, it may be used to treat almost any existing loudspeaker to achieve remarkably improved resu ts.

Furthermore, due to the ease in handling the material, it may be poured into a mold formed into the shape of a desired diaphragm, such as conical, exponential, hyperbolic, hemispherical or other suitable shape. The mold may be formed so that the flare has areas of equal or unequal thickness as desired, such as near the outer diameter or mouth thereof and near the apex where the voice coil is to be mounted. Furthermore, the mixture to be molded may be combined with selected quantities of fibrous materials such as paper pulp, wool pulp or other pulpy materials of the type generally used in paper cone speaker manufacture or combined with a selected quantity of woven or unwoven sheet material such as cloth or plastic screens of natural or snythetic origin so as to be molded integrally therewith, thereby forming a diaphragm with the backing support material distributed throughout the sound transmitting material.

While the present invention has been disclosed by means of specific illustrative embodiments thereof, it would be obvious to those skilled in the art that various changes and modifications in the method described or in the device or composition of matter, may be made without departing from the spirit of the invention as defined in the appended claims. For example, the present invention is not restricted to one ingredient for the major portion of the coating material and one ingredient for the binder material. If desired, several of the hard minerals such as selected proportions of pumice and gypsum may be mixed together to form the major portion of the coating material. Likewise, several of the binder materials such as selected proportions of polyvinyl acetate and melamine may be mixed together to form the binder for the hard mineral material.

What is claimed is:

1. An acoustic diaphragm having a self supporting body portion, at least one surface of said body portion being coated with a mixture of a hard powdered material and a soft adhesive binder selected from the group consisting of addition and condensation polymers, over 50% of the total dried weight of said mixture being said hard, powdered material, said coating having a thickness between .1 millimeter and 10.0 millimeters and a weight of between about .02. gram/ sq. cm. and 0.1 gram/ sq. cm.

2. The acoustic diaphragm of claim 1 wherein said material is a mineral.

3. The acoustic diaphragm of claim 1 wherein said material is a metal.

4. The acoustic diaphragm of claim 1 wherein said hard material includes powdered pumice and said adhesive binder includes polyvinyl acetate.

5. The acoustic diaphragm of claim 1 wherein said hard material includes about 75% alpha gypsum by weight and 25% melamine by weight, and said adhesive binder includes a polyvinyl acetate.

6. An acoustic diaphragm comprising paper having a coating thereupon consisting essentially of at least about 94% by weight of metallic particles and up to 6% by weight of a flexible binder, the thickness of said coating being about 1 to 1.5 millimeters and said coating having a weight of about .02 to .1 grams per square centimeter.

7. The acoustic diaphragm of claim 1 wherein said material is gypsum.

8. The acoustic diaphragm of claim 1 wherein said material is pumice.

9. The acoustic diaphragm of claim 1 wherein material is quartz.

10. The acoustic diaphragm material is plaster of Paris.

11. The acoustic diaphragm material is silica.

12. The acoustic diaphragm material is flint.

13. The acoustic diaphragm material is carbon 14. The acoustic diaphragm material is glass.

15. The acoustic diaphragm of claim 1 wherein said material is selected from the group consisting of aluminum and oxides and alloys thereof.

16. The acoustic diaphragm of claim 1 wherein said material is selected from the group consisting of iron and oxides and alloys thereof.

17. The acounstic diaphragm of claim 1 wherein said material is selected from the group consisting of coppe and oxides and alloys thereof. I

said

of claim 1 wherein said of claim 1 wherein said of claim 1 wherein said of claim 1 wherein said of claim 1 wherein said References Cited UNITED STATES PATENTS 1,924,803 8/1933 Paradise 181-32 2,842,138 7/1958 Billings et al. 128-91 2,874,066 2/1959 McLaughlin et al. 117-65 3,111,189 11/1963 Scholl 181-32 3,223,082 12/1965 Smith 128-91 3,256,223 6/1966 Heijmer 260-22 STEPHEN J. T OMSKY, Primary Examiner 

